Matriptase is a type II transmembrane serine protease expressed by cells of epithelial origin, including breast and prostate tumor cells. Matriptase is characterized by an N-terminal transmembrane domain and multiple extracellular domains, in addition to the conserved extracellular domain (Lin C. Y., et al., J. Biol. Chem., 1999, 274(26): 18237-18242.). Matriptase is a zymogen that needs to be activated by proteolytic cleavage to become a two-chain active enzyme. Under normal physiological conditions, there is excessive amount of endogenous inhibitor called HGF activator inhibitor-I (HAI-1) that binds to matriptase, tightly regulating the protease activity (Lin, C. Y., et al., J. Biol, Chem., 1999, 274: 18231-18236; Oberst, M. D., et al., J. Biol. Chem., 2003, 278:26773-26779.). Besides exerting an inhibitory function, HAI-1 also plays a critical role in activation, proper expression and intracellular trafficking of matriptase (Oberst, M. D., et al., J. Biol. Chem., 2003, 278: 26773-26779; Oberst, M. D., et al., Am. J. Physiol. Cell Physiol., 2005, 289:C462-C470.).
Matriptase is known to proteolytically activate the hepatocyte growth factor (HGF) and the urokinase plasminogen activator (uPA) and protease-activated receptor in vitro (Lee, S. L., et al., J. Biol. Chem., 2000, 275:36720-36725; Suzuki, M., et al., J. Biol. Chem., 2004, 279:14899-14908.). Both HGF and uPA have been implicated for their roles in cellular invasion and metastasis and in cellular motility (Trusolino, L. and Comoglio, P. M., Nat. Rev. Cancer, 2002, 2:289-300; Sidenius, N. and Blasi, F., Cancer Metastasis Rev., 2003, 22:205-222.). The cognate receptor for HGF is Met, a receptor tyrosine kinase. Upon the binding of HGF, Met can trigger multiple signaling pathways, including Ras-MAPK, PI3K, Src and Stat3, which eventually leads to invasive growth. High levels of Met combined with overexpression of matriptase are associated with a poor outcome for patients with breast cancer.
Studies of matriptase levels in several solid epithelial-derived tumors including breast, prostate and ovarian carcinomas have been performed in the past few years (for a review, see Uhland, K., Cell. Mol. Life Sci., 2007, 63(24):2968-78). A tissue microarray from patients with breast carcinoma showed that high-level expression of Met, matriptase and HAI-1 are associated with poor patient outcome (Kang, J. Y., et al., Cancer Res., 2003, 63: 1 101-1105.). In prostate tumors, increased levels of matriptase with decreased expression of HAI-1 were associated with increasing tumor grade (Saleem, M., et al., Cancer Epidemiol. Biomarkers Prev., 2006: 15, 217-227.). Overexpression of matriptase was also found in 82% of stage I II and in 55% stage III/VI of patients with ovarian cancer.
Given the significant roles of matriptase in tumor initiation, progression and metastases, several inhibitors for this protease have been investigated in animal models for their anticancer activity. Reduced tumor growth and metastasis formation by matriptase inhibitor, ecotin, has been shown in a PC-3 prostate carcinoma xenograft model (Takeuchi, N., et al., Proc. Natl. Acad. Sci. USA, 1999, 96, 11054-11061.). CVS-3983, another matriptase inhibitor, also reduced tumor size in a mouse model of androgen-independent prostate cancer (Galkin, A. V., et al., Prostate, 2004, 61, 228-235).
Monomethyl auristatin E (“MMAE”) is an FDA-approved synthetic antineoplastic agent that has shown promising use in chemotherapy based treatments. MMAE is a potent antimitotic compound but it exhibits high levels of cytotoxicity when administered alone, limiting its clinical value as a stand-alone compound. Monomethyl auristatin F (“MMAF”), also known as desmethyl-auristatin F, is an experimental synthetic anti-neoplastic agent that, like MMAE, is an antimitotic agent. Like MMAE, MMAF exhibits high levels of cytotoxicity when administered alone. Therefore, there is an urgent need for therapeutically effective antibody-based treatments that utilize MMAF and MMAE-bound immunoconjugates.